It used to be a case of you could create a new relation if you had room out of 5s
Now that is the short way I can explain it. The long way:
If a player has 5,10,15, 20,etc, etc relations you cannot create a new one. However if a player has a number in between you could create new relations up to the multiple of 5.for example if a player has 3 relations you can create 2 more to make 5. Once a new relation is made in game the you could go from 6 up to 10.
Not really edited relations much in fm13 so not sure it is still the case but it has been in previous editions.
If it is squad registration time you could always remove a players registration with a league to change 2md nationality. The will re register the player at a later date.
If it is for work permits then I find offering a contract. Then when it's accepted and work permit fails you appeal then change his nationality to an eu state (if signing for an eu club) and wait for the appeal date. On the appeal date you should get news that the appeal was successful. Accept transfer and change his nationality back to what it was. Voila easy work permit.